In aircraft, there are normally unavoidable discontinuities in the outer skin. Such discontinuities occur, for example, at the interface of adjacent panels and where access panels or doors are formed in the skin. In many situations, it is desirable to seal these discontinuities and/or provide a continuous electrical path along the outer surface of the skin across the discontinuities. One such situation is when the low electromagnetic signature integrity of the aircraft need to be preserved.
There are currently a number of known approaches to the problem of sealing movable and removable panels. One approach, illustrated in FIG. 1, is to use conductive putty 14 to fill a discontinuity 6 between the edge of a panel 4 and the portion 2 of the skin defining the opening in which the panel 4 is received. The conductive putty seal may be enhanced by covering it with a metallic tape 16. With or without the conductive tape 16, a conductive putty seal generally gives a very low signature. However, the use of the putty presents a number of problems. These problems include a time consuming installation procedure. The putty must be applied, cured for four to twenty-four hours depending on temperature, and sanded smooth. Another problem is that the putty tends to fail when the aircraft is in flight because the putty is hard after it cures and can crack and fall out if the panel moves. In addition, a conductive putty seal cannot be removed without destroying it by cutting or chipping it out. This removal procedure is also time consuming. Another problem is that the putty cannot be used on doors or panels that must be opened during flight.
Another known approach is to use a blade seal 18 which is attached to the panel 4 and sits over the top of the panel to skin gap 6, as shown in FIG. 2. This approach avoids the problems of time consuming installation and removal procedures and inability to move the panel when the seal is in place. However, there are a number of problems associated with the use of blade seals. Fasteners 20 are required to attach the seal to the panel, and rabbet cuts 21 on the panel are needed to make the seal essentially flush with the outer surface of the panel. Both the fasteners and rabbet cuts add to the weight of the panel by requiring increased thickness of the panel, such as additional layers of a composite material panel. Another problem is that a blade seal is difficult to mold around a panel corner. Blade seals have also been found to be relatively unreliable in maintaining the low signature integrity of the aircraft. The occurrence of electromagnetic scattering around blade seals at various points on the seal has been observed. In addition, blade seals have been known to lift up and thereby cause substantial electromagnetic problems. Another problem associated with blade seals is that they are very thin and sharp. This makes them subject to breaking off and dangerous to handle.
A third approach to the movable panel seal problem is the use of a conductive elastomer bulb seal, such as the seal 22 shown in FIG. 3. Such seals have the advantages of being formable into almost any shape and being relatively inexpensive. There are, however, drawbacks to the use of bulb seals. The seals tend to bulge up out of the gap if the panel and skin move relative to each other or if manufacturing tolerances result in variations in the gap width. The bulging usually increases the signature from the gap. Bulb seals are also difficult to form around panel corners. The effectiveness of bulb seals is limited because they create a step discontinuity between the seal and the side of the panel which in turn causes electromagnetic scattering.
IBM Technical Disclosure Bulletin, Vol. 22, No. 8A, January 1980 discloses compliant electromagnetic compatible shield for movable covers. The shield is for covers that slide longitudinally and is shown engaging the edge of a cover received into a three-sided recess. Wire brushes are mounted on opposite sides of the cover in the recess and are oriented at an acute angle to the cover. The ends of the bristles are trimmed at an angle. The differential force on the two sides of the cover created by the brushes keeps the cover laterally centered as it moves. The shield is described as providing "virtually complete metallic EMC effect".
U.S. Pat. No. 4,308,417, granted Dec. 29, 1981, to S. H. Martin, discloses an electromagnetic seal for a door or an aperture for wires. The seal includes a plurality of flexible filaments. The majority of the filament's are insulative, with the rest being conductive and interspersed among the insulative filaments. In the door seal application, the filaments are lightly compressed when the door is closed. In the application for sealing a hole in an enclosure wall, a pair of sealing devices is mounted on the wall on opposite sides of the hole. The filaments extend across and cover the hole. The wires pass through the flexible filaments. The ends of the filaments intermingle.
Swiss Pat. No. 457,574, published Aug. 15, 1968, shows an apparently similar system for sealing an aperture in which opposing brushes laterally abut each other.
Sealing arrangements for electromagnetic or radio frequency shielding of a door structure are disclosed in U.S. Pat. No. 3,507,974, granted Apr. 21, 1970, to D. B. Clark et al.; U.S. Pat. No. 3,518,355, granted June 30, 1970, to C. T. Luce; U.S. Pat. No. 4,177,353, granted Dec. 4, 1979, to R. G. McCormack; and U.S. Pat. No. 4,371,175, granted Feb. 1, 1983, to G. C. Van Dyk, Jr. Each of these shielding arrangements includes an inflatable bladder. In the Clark et al system, a bladder is inflated to flex a finger stock, resembling an elongated leaf spring, into engagement with the door frame. In the Luce system, a bladder is inflated to urge the door against a seating surface. The McCormack system is similar to the Luce system and includes a bladder with a conductive coating. Van Dyk, Jr. discloses an inflatable gasket inside a flexible conductive shielding mounted on the door frame.
The devices discussed above and the prior art cited in the above patents should be studied for the purpose of putting the present invention into proper perspective relative to the prior art.